Prior to the present invention, somewhat analagous self-pressurized portable containers have been the subject matter of the U.S. Pat. No. 3,895,742 to Wulbern, and U.S. Pat. Nos. 3,989,170 and 3,929,262 and 3,880,323 to Viehe. While each of the U.S. Pat. Nos. 3,989,170 and 3,880,323 disclose that a central portion of the circular expandable elastic membrane is thicker than its peripheral portions becoming uniformly thinner radially outwardly from a central location of the membrane, the commercial industry has nevertheless been plagued with continued ruptures of the membrane. There is an obvious limit as to how thick the entire circular elastic membrane may be in order to remain operative in its required sufficient degree of elasticity as to permit it to baloon as a water container as water is under pressure forced into the enclosure beneath the membrane. It is apparent that if the thickness of this membrane is too great, that such excessive thickness will result in and in fact increase the probability of rupture when sufficient liquid pressure forces expansion thereof. Moreover, the portable containers of the above-noted type are high susceptible to rupture when exposed to sunlight over a period of extended usage, as the present Applicant has found to be the result of ozone deteriorating effect on the elastic membrane. Moreover, the present applicant has discovered that many elastic or rubber membranes are highly susceptible to rupture, and Applicant has discovered improved composition for the manufacture of these type of vessels.